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Abstract
A hybrid cascaded configuration consisting of a fiber Sagnac interferometer (FSI) and a Fabry-Perot interferometer (FPI) was proposed and experimentally demonstrated to enhance the temperature intensity by the Vernier-effect. The FSI, which consists of a certain length of Panda fiber, is for temperature sensing, while the FPI acts as a filter due to its temperature insensitivity. The two interferometers have almost the same free spectral range, with the spectral envelope of the cascaded sensor shifting much more than the single FSI. Experimental results show that the temperature sensitivity is enhanced from −1.4 nm/°C (single FSI) to −29.0 (cascaded configuration). The enhancement factor is 20.7, which is basically consistent with theoretical analysis (19.9).
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In past decades, optical fiber temperature sensors have been intensively studied due to their many intrinsic advantages such as electrically passive operation, remote sensing capability and immunity to electromagnetic interference [1–6]. Several configurations have been employed, such as fiber Bragg gratings (FBGs) [7,8], long period fiber gratings (LPFGs) [9,10] and optical fiber interferometers (OFIs). All of them, FBG-based temperature sensors have relatively low sensitivity of ~10 pm/°C [8]. LPFG-based temperature sensors are cross sensitive to surrounding materials and fiber bending. OFIs including Fabry-Perot interferometers (FPIs) [10–13], Mach-Zehnder interferometers (MZIs) [14–16] and fiber Sagnac interferometers (FSIs) [17,18] are good candidates for highly sensitive temperature sensors.
Vernier-effect is an efficient method to enhance the accuracy of measurement instruments and is widely used in calipers and barometers. Recent years, it has also been applied in photonic devices by using two interferometers respectively as fixed and sliding parts of the Vernier-scale. Several configurations are proposed to achieve the Vernier effect, such as two cascaded FPIs [19,20], two cascaded MZIs [21], two cascaded fiber loops [22], and two cascaded FSIs [23]. All above cascaded configurations are formed by the same kind of interferometers. For this kind of configuration, it is generally difficult to make one interferometer (as fixed part of the Vernier-scale) to be insensitive while the other (as sliding part of the Vernier-scale) to be sensitive to the measured parameter. In fact, it can be achieved by combining two different interferometers. Recently, Troia et al. have proposed the combination of a ring resonator and a MZI for Vernier-effect refractive index sensing [24]. In the configuration, only the MZI is sensitive to refractive index.
In the paper, we have proposed a hybrid cascaded configuration which consists of a FPI and a FSI. The FPI is intrinsically well shielded from environment temperature change due to the short length of the air cavity, so it act as the fixed part of the Vernier-effect. The FSI is sensitive to the temperature and uses as the sliding part of the Vernier-effect.
This paper has the following outline. In Section 2, operation principle of the hybrid cascaded configuration is introduced. Section 3 is devoted to the experimental analysis. Section 4 summarizes the results.
2. Operation principle and simulation
The schematic diagram of the temperature sensor based on the hybrid cascaded configuration of a FPI and a FSI is shown in Fig. 1. The FSI connects with the FPI via a 3dB fiber coupler and a fiber circulator, and in the fiber Sagnac loop there is a certain length of Panda fiber.
Fig. 1 Schematic diagram of the temperature sensor based on hybrid cascaded configuration of a FSI and a FPI (OSA: Optical spectral analyzer).
The reflection spectrum function of the FPI can be expressed as:
where α is the transmission loss in mirror M2; R1 and R2 are the reflection coefficients of mirrors M1 and M2, respectively; L1 is the length of the FP cavity; n = 1 is the refractive index of the air filling in the cavity; λ is the optical wavelength in vacuum. And the free spectral range (FSR) of FPI isThe transmission spectrum of the Sagnac fiber loop is approximately a sinusoidal function of wavelength, which can be described by the following equation:
where is the phase shift between the two polarization modes; B and L are the birefringence coefficient and length of the Panda fiber, respectively. If the condition (m is an integer and represents the fringe order) is fulfilled, the transmission spectrum will reach its maxima. Therefore, the dip wavelength of the mth order is given byWe can derive the FSR between the two adjacent dips asWhen the temperature changes, the birefringence change ΔB of the Panda fiber will lead to the spectral shift, which can be expressed as follows:We can simulate the outputs of the single FSI and the single FPI, and obtain the results shown in Fig. 2(a) (FSRFSI = 3.2 nm and FSRFPI = 3.0 nm). The outputs are just like scale rulers and the scales of the rulers are as wide as the FSRs. Thus we can improve the sensitivity by constructing a Vernier-scale with the cascaded FSI and FPI as shown in Fig. 1. The Vernier-scale, which consists of two scales with different periods, is widely used in calipers and barometers to enhance the accuracy of measurement instruments. Recent years, it has also been applied in photonic devices. The overlap between two scale lines is employed to perform the measurement. The total output spectrum of the cascaded configuration is the product of the individual ones, which exhibits peaks at wavelengths where interference peaks of the FSI and the FPI partially overlap, and the height of each of these peaks will be determined by the amount of overlap. The highest peak of the total output occurs when the peaks of Sagnac loop and FP cavity are at the same wavelength. All the peaks form the spectral envelope and the FSR of the envelope is given by
In our simulation, FSRenvelope is 48.0 nm and the highest peak is at 1548.0 nm as shown in Fig. 2, which is consistent with above theoretical analysis.In practice, we can manage the lengths of Panda fiber and FP cavity to get the ideal FSRs. The air FP cavity is well shielded from environment temperature change due to the short length and the low thermal expansion, so it acts as the fixed part of the Vernier-scale. The FSI is for the sliding part of the Vernier-scale, as the temperature changes will cause a shift of the interference wavelengths. When the temperature changed, spectra of the FSI will shift. Then the shift of spectral envelope is magnified by a certain factor. And the enhanced factor is decided by
So the envelope shift versus temperature can be shown as
If the FSI shifts with a difference value of the two FSRs of the FSI and the FPI, the highest peak will hop to the adjacent peak. And when the FSI shifts with a value of FSRFSI, the highest peak (or the dip of spectral envelope) will shift a value of FSRenvelope.
The simulation result of Vernier-effect is shown in Fig. 3. FSRFPI are set to be 3.0 nm. When the shift of single FSI is 0.5 nm, the shift in the envelope is magnified to 8.0 nm. The sensitivity is enhanced by 16 times which is consistent with the theoretical analysis.
3. Experiment
As shown in Fig. 1, in the experiment, an ASE source (C and L band) with ~80 nm wavelength range is used as an input light source. The cascaded configuration is formed by a fiber Sagnac loop and a FP cavity. The length of the Panda fiber is 2.49 m with the birefringence B = 3.0 × 10−4. The FP cavity is fabricated by splicing a section of fiber tube between two sections of single mode fiber. The two reflective mirrors, M1 and M2, form an air-filled FP cavity with the length of 355 μm. The light is firstly passes through the Sagnac loop, then is reflected by the FP cavity, and finally is received by optical spectral analyzer (OSA) with a resolution of 0.02 nm. Figure 4 illustrates the spectrum of the single FSI and the cascaded FPI and FSI (42 °C). FSRFSI and FSRFPI are measured to be 3.21 nm and 3.38 nm, therefore the calculated enhancement factor is 19.9.
The temperature characteristics of the single FSI and the cascaded FSI and FPI are tested by placing the sensors in a temperature controlled furnace with the temperature range from 42 °C to 44 °C. For clear comparison of the two kinds of sensors, Fig. 5 shows the spectra of the individual Sagnac sensor and the hybrid cascaded sensor of FSI and FPI at the temperatures of 42.2 °C and 43.0 °C. Both of the two sensors have a blue wavelength shift with temperature increasing. Obviously, the cascaded sensor has a much higher wavelength shift (−23.0 nm) than the individual FSI (−0.8 nm). The temperature sensitivity of the two kinds of sensors are shown in Fig. 6, which are −29.0 nm/°C and −1.4 nm/°C, respectively. So the experimental enhancement factor is 20.7, which is basically consistent with the theoretical one (19.9). The difference between the theoretical and experimental results is mainly caused by the determination of the dip positions in the spectral envelope. In fact, peaks fitting method is used to seek the exact position of spectral envelope dips.
Fig. 5 Spectral shifts of (a) single FSI and (b) cascaded FSI and FPI at the temperature of 42.2 °C and 43.0 °C.
The sensitivity of our sensor can be enhanced much more by choosing FSI and FPI with smaller difference in FSR and in theory the sensitivity can be increased indefinitely. However, we must consider the number of envelopes in a certain wavelength window. In addition, the length of Sagnac loop can be shorten by replace the Panda fiber using higher birefringence fibers.
4. Conclusion
In conclusion, a novel temperature sensor based on Vernier-effect was proposed and experimentally demonstrated with ~20 times sensitivity enhancement. The Vernier-effect is achieved by a hybrid cascaded configuration consisting of a fiber Sagnac interferometer (FSI) and a Fabry-Perot interferometer (FPI). FSI, which consists of a certain length of Panda fiber, is for temperature sensing, while the FPI acts as a filter due to the temperature insensitivity. Experimental results show that the sensor based on hybrid cascaded configuration has a high sensitivity of −29.0 nm/°C which is 20.7 times as high as that of single FSI, which is basically consistent with the theoretical analysis.
Funding
Heilongjiang Natural Science Foundation (No. F2017012); National Natural Science Foundation of China (Nos. 51777046, 51672062, 51575149, 11574065, 61775053, 61378029); the Science Foundation for Outstanding Youths of Heilongjiang Province (JC2016016); Harbin Research Fund Technological Innovation (2016RQQXJ108).
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